Keyword
additiveIf declare(f,additive) has been executed, then:
(1) If f is univariate, whenever the simplifier encounters f applied
to a sum, f will be distributed over that sum. I.e. f(y+x) will
simplify to f(y)+f(x).
(2) If f is a function of 2 or more arguments, additivity is defined as
additivity in the first argument to f, as in the case of sum or
integrate, i.e. f(h(x)+g(x),x) will simplify to f(h(x),x)+f(g(x),x).
This simplification does not occur when f is applied to expressions of
the form sum(x[i],i,lower-limit,upper-limit).